In conventional electronic automatically controlled vehicular heating and air conditioning systems, it has been the practice for many years to include an air temperature sensing device for maintaining the interior air temperature at regulated preselected levels. This device is normally located behind the dash, reasonably close to the uppermost areas of the dashboard. Interior air is drawn through a vent in the dash and then through the air temperature sensor by means of establishing a negative pressure at the downstream end of the sensor. This is usually accomplished by connecting an air line from the outlet end of the sensor to a venturi valve. One such arrangement is shown in U.S. Pat. No. 2,316,619 issued to Raney, wherein the venturi valve is located within the downstream tube leading from the temperature sensor with air from the positive pressure end of a blower being passed through the venturi to create the area of negative pressure in communication with the sensor outlet.
Another arrangement known in the prior art is to connect a similar venturi valve device to the positive pressure end of a blower housing. Each such system is fairly similar and each would apparently work equally well provided the positive pressure is controlled by the outlet of the blower. However, under certain conditions, namely at low blower speeds and vehicle speeds in excess of 15 to 20 miles per hour, the ambient air introduced into the system is at a higher pressure than the blower output, which destroys the negative pressure across the temperature sensor outlet, thereby rendering the air temperature sensing system inoperative.